Safe, efficient and economical operation of a motor vehicle depends, to a significant degree, on maintaining correct air pressure in all (each) of the tires of the motor vehicle. Operating the vehicle with low tire pressure may result in excessive tire wear, steering difficulties, poor road-handling, and poor gasoline mileage, all of which are exacerbated when the tire pressure goes to zero in the case of a “flat” tire.
The need to monitor tire pressure when the tire is in use is highlighted in the context of “run-flat” (driven deflated) tires, i.e., tires which are capable of being used in a completely deflated condition. Such run-flat tires, typically designed for the passenger vehicle market, as disclosed for example in commonly-owned U.S. Pat. No. 5,368,082, may incorporate, for example, reinforced sidewalls, mechanisms for securing the tire bead to the rim, and/or a non-pneumatic tire (donut) within the pneumatic tire to enable a driver to maintain control over the vehicle after a catastrophic pressure loss, and are evolving to the point where it is becoming less and less noticeable to the driver that the tire has become deflated. The broad purpose behind using run-flat tires is to enable a driver of a vehicle to continue driving on a deflated pneumatic tire for a limited distance (e.g., 50 miles, or 80 kilometers), and at a limited speed, prior to getting the tire repaired, rather than stopping on the side of the road to repair the deflated tire. Hence, it is generally desirable to provide a low tire pressure warning system within the vehicle to alert (e.g., via a light or a buzzer) the driver to the loss of air pressure in a pneumatic tire.
To this end, a number of electronic devices and systems are known for monitoring the pressure of pneumatic tires and providing the operator of the vehicle with either an indication of the current tire pressure, or alerting the operator when the pressure has dropped below a predetermined threshold level. It is known that the air pressure within any closed space such as a pneumatic tire (with a relatively fixed volume) varies with the temperature of the air according to the “ideal gas law”. Therefore, many tire pressure monitoring systems incorporate means of measuring the air temperature within the tire and then utilize that temperature measurement to adjust the pressure measurement appropriately. The adjustment, or temperature compensation, of the pressure measurement may be done by the electronic device before reporting (transmitting) the temperature-compensated pressure reading to a monitoring system receiver, or else both the temperature and pressure readings are reported to the receiver. In the latter case, the monitoring system receiver may perform the temperature- compensation, or it may simply record and/or report the temperature reading along with the pressure readings.
Tire monitoring systems are also utilized in truck tires and in much larger tires such as tires for Off-The-Road (OTR) vehicles. In order to transport bulk materials, such as coal, iron ore and other minerals, the mining industry, for example, uses OTR vehicles that typically weigh up to 250 tons or more when fully loaded, with the result that exceedingly high internal stresses are imposed on the tires of such vehicles in the course of their daily use. Such internal stresses, which are primarily attributable to a number of factors including driving at excessive speeds, are so destructive of such tires that it is not uncommon to have to replace the tires. On the other hand, in order to maximize the productivity of OTR vehicles, they are normally driven as fast as possible until a user perceives that the internal physical condition of any given tire is marginal. Whereupon, the operator either stops, in the case of a loss of tire pressure, or reduces the speed of the vehicle, in the case of an excessive temperature condition, to relieve the internal stresses giving rise to the marginal condition, thereby prolonging the life of the tires. Thus the speed of an OTR vehicle is controlled on the basis of the operator's perception of the condition of the tires at any given time. And, if the operator's perception is erroneous, the productivity of the vehicle is unnecessarily reduced. Accordingly, tire monitoring systems are desirable to ensure that the operators of OTR vehicles are provided with accurate information concerning various conditions of the tires of such vehicles, with a view to maximizing the productivity of the vehicles. For these large OTR vehicle tires, tire condition (short of a deflation) is indicated not only by pneumatic pressure, but also by temperatures at critical locations in the tire carcass where the temperature is most closely indicative of a marginal condition signaling an impending breakdown of the tire. For example, it is known for monitoring devices to sense tire temperature at the side edges of the belts, and thus near the shoulder portions of tires, where delaminations of the ply, belts and surrounding rubber materials occur due to the build up of internal stresses.
Due to the difficulty of communicating temperature and pressure sensed within a rotating tire, many of the known electronic devices include transmission capability, typically by radio frequency (RF) means, so that the electronic device is capable of transmitting an RF signal indicative of the tire pressure (and optional other parameters such as temperature) to a remotely-located receiver. Additionally, the electronic device which monitors the tire may have the capability of receiving as well as transmitting signals, in which case the electronic device is referred to as a “transponder”.
As used herein, a “transponder” is an electronic device capable of receiving and transmitting signals (typically radio frequency), and impressing variable information (data) in a suitable format upon the transmitted signal indicative of a measured condition (e.g., tire pressure) or conditions (e.g., tire pressure, temperature, revolutions), as well as optionally impressing fixed information (e.g., tire ID) on the transmitted signal, as well as optionally responding to information which may be present on the received signal. “Passive” transponders are transponders powered by the energy of a signal received from an “interrogator”, which is a receiver capable of sending a signal to the transponder. “Active” transponders are transponders having their own power supply (e.g., a battery). As used herein, the term “tag” refers either to a transponder having transmitting and receiving capability, or to a device that has only transmitting capability. Generally, tags which are transponders are preferred in vehicular tire monitoring systems. As used herein, the term “tire-pressure monitoring system” (TPMS) or even more simply a vehicular “tire monitoring system” indicates an overall system comprising tags within the tires and a receiver which may be an interrogator disposed within the vehicle.
Choosing between active and passive transponders for use in a particular application is a complex decision-making process. Passive devices are limited by the amount of power they can receive from an interrogator, and the power is limited by considerations of interrogator placement and distance, and by the types of antennas which can be employed in the tire and on the vehicle, to name but two of the factors. Active devices which are battery powered are limited, for example, by the amount of power and energy available from the battery included with the tag, and thus by the life of the battery. It is also known to provide active tag power by including a miniature electric generator within the tire hub, to conduct power to the tire hub through slip rings, etc., but the present invention is addressed primarily to active tags which are battery powered. Obviously, the battery must live long enough to avoid the nuisance of frequent battery changes, and preferably outlives the operational life expectancy of the tire. Thus, conserving energy is an issue for battery powered tags.
The term “battery” is used herein to loosely refer to any type of stored electrical energy device, whether composed of one or more than one cells.
As used herein, the terms power and energy are mainly intended to refer to the overall concept of electrical energy which is available for use by a tag. In a battery powered tag, a limited quantity of electrical energy is stored in a battery, and the battery's energy level is reduced as the tag utilizes the energy to operate the active components of the tag. Although in technical terms, electric “power” is defined as a rate of use of electrical “energy”, the term “power” may also be used herein in the common sense of “strength”. Thus the strength remaining in a battery may be referred to herein as, for example: battery power, power level, energy level, remaining energy, and the like.
It is known to mount a tag, and associated condition sensor(s) (e.g., pressure sensor and/or temperature sensor(s)) within each tire of a vehicle, and to collect information from each of these transponders with a common single interrogator (or receiver), and to alert a driver of the vehicle to a low tire pressure condition requiring correction (e.g., inflating or replacing the tire). For example, U.S. Pat. No. 5,540,092 (Handfield, et al.; 1996), discloses a system and method for monitoring a pneumatic tire. FIG. 1 therein illustrates a pneumatic tire monitoring system (20) comprising a battery powered transponder (22) and a receiving unit (24). Each transponder has an identification code and is programmed to identify which wheel of the vehicle it is installed upon. A stated object of the Handfield invention is “to provide an ultra-low-power, efficient transmitter for use in a pneumatic tire monitoring system so as to significantly extend the useful battery life.” As stated in the Handfield background: “To increase battery life, some prior art devices transmit a signal only when improper inflation is detected. Other devices use the centrifugal force created by rotation of the tire to activate the tire monitoring device. These types of systems allow a window of opportunity for undetected damaging operation to occur. For example, a centrifugally-actuated system would not alert an operator to a flat tire until the vehicle reached a predetermined operating speed. . . . A system which only detects a flat tire would allow operation of an improperly inflated tire which was not yet flat but which could lead to premature tire wear requiring replacement.”
Other prior art approaches to battery energy conservation in active tags include transponders which remain in a “sleep” mode, using minimal power, until “woken up” by a signal from an interrogator, or by an internal periodic timer, or by an attached device. These approaches still potentially “waste” battery power by checking and reporting tire conditions unnecessarily often. An example is seen in U.S. Pat. No. 5,562,787, issued to Koch et al., wherein a method and apparatus for monitoring conditions in the tires of vehicles is disclosed. The apparatus comprises an active self-powered programmable electronic device which is installed in or on the interior surface of a pneumatic tire or on a tire rim. This device can be used for monitoring, storing and telemetering information such as temperature, pressure, tire rotations and/or other operating conditions of a pneumatic tire, along with tire identification information. The device includes a power source which can be placed into an active or a dormant mode using a data band switching circuit. The device can be activated by externally transmitted radio frequency waves or microwaves and in response, the device compares or transmits information and provides a warning in the event a preselected limit is exceeded.
It is an aspect of the present invention to provide improved battery energy conservation for battery powered active tags, especially those that are used in tire monitoring systems.